The present invention relates to rare earth activated phosphors. Such phosphors are known to possess excellent light output and colour rendering properties and have been utilised successfully in many display technologies. One particularly successful material, europium activated yttrium oxide (Y2O3:Eu3+) has shown particular promise in the field of field emission displays.
There is a need for low voltage phosphors for use in flat panel displays or FED type (field emission) displays. By lowering the voltage one will lower the cost of the driving electronics used to control image generation. For this purpose very small particle sizes, of the order of a few nanometres, are needed. However, a reduction in particle size of a phosphor such as europium activated yttrium oxide results in a decrease in the luminescence efficiency. This is caused by nonradiative recombination via surface defect states. If, though, special steps are taken during the preparation of the particles, these surface states can be passivated thus giving rise to nanoparticle sizes of the phosphor having efficient light emission. However, the special steps which are needed to prepare nanoparticles (i.e. xe2x89xa6100 nm) of conventional phosphors such as europium activated yttrium oxide are complex with the result that the cost of producing them is high. In addition it is difficult to be able to produce such particles on a significant scale.
There is, therefore, a need for a way of producing phosphors in the form of nanoparticles which avoids these production difficulties.
It has now surprisingly been found according to the present invention that nanoparticles can readily be obtained of a new family of rare earth oxides, these being such that the activating ion is of the same element as that of the oxide. Accordingly, the present invention provides a compound of the formula Z2O3:Zx+ where Z represents a rare earth element and x is 2 to 4, generally 3.
Preferred rare earth elements/metals include terbium, europium and cerium.
Preferably the phosphors are in the form of nanoparticles, especially ones having a particle size from 2 to 10 nm, especially not exceeding 5 nm, for example about 4 nm.